Onium hydroxides, such as tetramethylammonium hydroxide, are used in semiconductor processes. Such onium hydroxides are required to be of extremely high purity. For example, the SEMI C46-0699 specification for aluminum ion requires that the aluminum content be less than 10 part per billion (ppb) in 25% tetramethylammonium hydroxide.
Environmental and economic concerns drive the desire to recycle materials, including materials such as the onium hydroxides used in semiconductor processes. Materials such as onium hydroxides and onium salts are the subject of rather stringent environmental regulations. The production of new onium hydroxides and onium salts is more expensive than is the recycling of used onium hydroxides and onium salts. However, use of materials such as onium hydroxides in semiconductor processing inevitably contaminates the onium hydroxides with materials such as aluminum and other metal ions. While removal of most metal ions, including aluminum, down to levels in the part per million (ppm) range has been known, it has proven quite difficult to remove metals such as aluminum to levels near and below 1 ppb. In order to meet this specification, it has been generally necessary to simply start with new onium hydroxide material, since it has been impossible to attain such low levels of metal ions in recycled onium hydroxide solutions. Thus, it has been difficult to recycle materials such as the onium hydroxides used in semiconductor processing for re-use in semiconductor processing. Removal of metals such as aluminum from recycled onium hydroxide and salt solutions has been a long-standing problem, and a need remains for such methods, if materials such as onium hydroxides and salts are to be successfully and economically recycled for use in the most demanding applications, such as in semiconductor processing.